Method of preparing copper-plate printing plates for printing

ABSTRACT

AN ENGRAVED PRINTING PLATE HAVING AN INK RETAINING SURFACE OF COPPER IS PREPARED FOR PRINTING BY APPLYING A LAYER OF A MATERIAL SUCH AS MATT CHROME WHICH IS REPELLENT TO INK BUT ACCEPTS A WETTING AGENT OVER ALL OF THE SURFACE. A PROTECTIVE COATING IS APPLIED TO THE NON-ENGRAVED PART OF THE SURFACE. THIS COATING MAY BE POWDERED BITUMEN APPLIED TO A FILM OF PRINTING INK THEN FUSED BY HEAT TREATMENT. THE LAYER OF CHROME IS THEN REMOVED FROM THE ENGRAVED PART OF THE SURFACE BY CHEMICAL OR ELECTRO-   LYTIC MEANS AND THE PROTECTIVE COATING IS THEN REMOVED BY SOLVENTS.

June 26, 1973 lam 3,741,835

METHOD OF PREPARING COPPER-PLATE PRINTING PLATES FOR PRINTING FiledMay 1. 1970 'IIIIIIIIII/l 'IIIIIIIIIIIIIIIIIIIIIIIIIIIll United StatesPatent US. Cl. 15614 4 Claims ABSTRACT OF THE DISCLOSURE An engravedprinting plate having an ink retaining surface of copper is prepared forprinting by applying a layer of a material such as matt chrome which isrepellent to ink but accepts a wetting agent over all of the surface. Aprotective coating is applied to the non-engraved part of the surface.This coating may be powdered bitumen applied to a film of printing inkand then fused by heat treatment. The layer of chrome is then removedfrom the engraved part of the surface by chemical or electrolytic meansand the protective coating is then removed by solvents.

The present invention relates to processes for the preparation ofcopper-plate printing plates the engraved lines of which retain ink andthe surface of which apart from the lines repels ink and retains awetting agent. Such printing plates can be used in a copper-plateprinting machine which works without wiping devices which have been usedup to the present time. For this purpose, a device for applying wettingagent to the plate is situated before the conventional inking device.This wetting agent covering the surface of the plate apart from theengraved lines prevents the surface of the said plate from being inked,in such a way that during its passage by the inking device, the ink isonly applied to the engraved lines.

According to a process suggested by the applicant (Swiss patentapplication No. 18,526/68) starting from an engraved plate with anink-repellent material forming a layer covering the non-printing surfacethereof, the engraved lines are filled with a material which repelsmetallic deposits after which a layer of a second material repellent toink, but acceptable to a wetting agent, is then applied on the surfaceof the printing plate and, finally, the material filling the lines isremoved.

The present invention concerns a process in which it is possible toeliminate filling the engraved lines with a filling material. Accordingto the invention, there is provided a method for preparing a printingplate for printing, comprising providing an engraved printing platehaving a surface in ink retaining material, applying a layer of a secondmaterial repellent to ink but accepting a wetting agent on said surface,and removing said layer from the engraved part of said surface.

After the layer of second material is deposited, the surface of theprinting plate excluding the engraved part is preferably coated with aprotective coating which is removed after the removal of the secondmaterial. The second material is preferably matt chromium and is removedfrom the engraved part by chemical or electrolytic means, the potectivelayer not being affected by this treatment. Advantageously, theprotective layer is held by means of a coating of glue previouslyapplied to the surface of the non engraved part of the printing plate,this coating of glue conveniently consisting of a conventional printingink applied by means of an inking cylinder. The protective layer,cosisting of powdered bitumen is next 3,741,835 Patented June 26, 1973applied to the damp coating of ink, fused by the application of heatand, after the removal of the second material from theengraved part, theprotective layer and the ink are removed from the surface of theprinting plate.

In order to improve the property of the second material for retaining awetting agent, it is recommended that the plate is treated with anelectrolytic aqueous solution containing ions which penetrate, forexample by capillary action into the porous structure of the secondmaterial.

In such a manner, the outer layer forming the nonprinting surface,preferably of matt chrome, is additionally impregnated with constituentscapable of retaining the wetting agent and repelling ink.

A method of implementing the process according to the invention will nowbe described with reference to the accompanying drawing which is adiagrammatic crosssection through a printing plate at an intermediatestage of the method according to the invention.

The printing plate is indicated by the general reference numeral 1. On acopper plate 14 with engraved lines 15 is applied a layer 16 of chromiumwhich has ink-repellent properties when covered by a wet film. Thischromium layer can be deposited by chemical or electrolytic means.

Next, the non-printing surface of plate 14, i.e. the part not providedwith engraved lines, is covered with a protective coating which is notaffected by the later treatment for removal of the chromium layer 16from the lines. For this purpose, the surface of the plate not havingengraved line-s 15 is covered with an adhesive layer 25, composed of aconventional printing ink or another similar material, by means of aninking cylinder. Care should be taken to prevent the ink from enteringthe lines 15. On the still damp coating 25 the so-called protectivecoating 26 is spread in powder form. Bitumen is preferably used for thispurpose. The excess bitumen or other material used is shaken ofi and/orremoved by compressed air, the powder being particularly carefullyremoved from the engraved lines. By suitable heating of the plate oronly the surface thereof, the powder is brought to its fusion point sothat it forms a solid bituminous layer 26. Simultaneously, the adhesivecoating is dried in such a way that the protective coating only adheresto the surface of the plate where it is not provided with lines 15.

This protective coating is then capable of resisting a caustic orelectrolytic treatment for removing the deposit of chromium 16 from thelines. During this treatment it is advantageous to ensure that all thechrome is removed but that the under-lying copper is not affected.

The next step consists of removing the protective coating from thesurface of the plate. For this purpose the bitumen is firstly removed bytreatment with naphtha or naphthalene, for example, after which theunderlying coating of ink or adhesive 25 is dissolved by means of asuitable solvent such as phenol. The protective coating can also beremoved by means of trichlorethylene. The finished printing plate isthus covered with a layer of chromium 16, which repels ink but retainsthe wetting agent, on the non-printing surface thereof which does nothave any engraved lines, whilst the engraved lines are in copper whichis an ink retaining material.

In order to further improve the properties of the chromium layer toretain the wetting agent, it is recommended that the plate be treatedafter the chromium deposit with an electrolytic aqueous solution,preferably brought to a temperature of between 50 C. and C., and whichcontains ions which penetrate the porous structure of the chromiumespecially by capillary action, and which increase the desired propertyof retaining a wetting agent. In this way, the chromium is additionallyimpregnated with constituents capable of retaining a wetting agent andrepelling ink and which are attached to the chromium by chemicalreaction.

As the electrolytic solution, it is preferable to use a wetting agent,the composition of which is similar to that which is applied duringprinting for wetting the surface of the plate: such agents are known,for example, in offset printing methods. This electrolytic solution maybe al kaline and contain phosphates, for example.

In order to improve this effect even more it is advantageous to add anadhesive such as gelatine or gum arabic to the electrolytic solution.Thus a layer having good wetting agent retaining properties which aremaintained over a period of time is obtained. When the treatment takesplace before the application of the protective coating, thetrichlorethylene applied to remove this layer does not affect thisimpregnation because trichlorethylene has only physical solubilityeffects.

On the other hand, treatment of the plate with this electrolyticsolution can only be made after removal of the protecting coating.Consequently, the possible traces of trichlorethylene or other solventsemployed for the removal of this protective coating are removed from thechromium by the constituents of the electrolytic solution so as toensure the complete removal of the trichlorethylene or other solventused from the chromium.

It has been found in practice that using the process according to theinvention it is possible to manufacture copper printing plates havingengraved lines with a depth of up to 0.05 mm. for example.

As the mechanical resistance of copper used for these printing plates islow, the plate itself, designated by 14, is made of nickel, engravedwith lines, on which a layer of copper is applied, for example bycondensation in a vacuum before carrying out the above-describedprocess. A copper layer 0.0025 mm. thick is sufficient. The treatment ofthe plate is carried out as described hereinabove. After removal of thechromium 16 from the engraved lines, the underlying layer of copper,which is not affected by the treatment, forms an ink-retaining surface.

The invention is not limited to the described embodiment. As far asconcerns the material which accepts or repels ink and retains thewetting agent, this can be any substance which has these properties andwhich can be deposited on the printing plate. The protective layer canalso consist of any suitable material which resists the treatments forthe removal of the ink-repellent material from the engraved lines. Thislatter treatment can be carried out by any known mechanical, chemical orelectrolytic method or even an electrical process such aselectro-erosion, or by a combination of such methods.

For the removal of the protective layer, other known mechanical,chemical or electro-chemical methods can be used, and this treatment canbe followed by a polishing operation to finish the surface of the plate.

It is possible to eliminate the application of a protective layer 25 and26 if, after application of the ink-repellent but wetting agentretaining layer and, possibly, after treatment with an electrolyticsolution, this the protective layer is removed from the engraved linesby a suitable treatment, without damaging the layer of the materialcovering the non-printing surface of the plate not provided withengraved lines.

What is claimed is:

1. An improved method for preparing an engraved copper faced printingplate which comprises:

(a) applying to the entire surface of the engraved copper faced plate aprimary layer of chromium which is repellent to ink but readily acceptsa wetting agent,

(b) adhesively applying a protective coating to the nonengraved portionof the surface of the plate comprising ink and powdered bitumen,

(c) removing the chromium layer from the engraved portions of theprinting plate,

(d) removing the protective layer of ink and bitumen from thenonengraved portion of the plate,

(e) treating the chromium layer with an aqueous electrolytic solution ofan alkaline phosphate and an adhesive for improving the ink repellentand wetting agent retaining characteristics of the chromium layer.

2. A method according to claim 1, wherein said chromium layer is removedas in step (c) by chemical or electrolytic means.

3. A method according to claim 1, wherein in step (b) a printing ink isapplied to the non-engraved surface already coated with chromium andpowdered bitumen is applied over the printing ink, the ink and bitumenfused and in step ((1) this coating of fused ink and bitumen is removedby treatment with a first solvent to remove the bitumen and a secondsolvent to remove the printing ink.

4. A method according to claim 1, wherein the electrolytic solutioncontains an adhesive selected from the group consisting of gelatin orgum arabic.

References Cited OTHER REFERENCES Apps: Inks for the Major PrintingProcess, (1963), p. 134 cited.

Abraham: Asphalts & Allied Substances (11945), p. 258 cited.

A Ruemer: Inland Printer/Amer. Lith., pp. 90, 92, June 1965.

Diehl: Inland Printer/Amer. Lith., pp. 90, 92, August 1966.

Reid: Offset Platemaking, Lithographic Tech. Foundation, pp. 152-156,184; pub. December 1959.

GEORGE F. LESMES, Primary Examiner R. J, ROCHE, Assistant Examiner US.Cl. X.R.

